a. Field of Invention
This invention relates to a pipe joint connecting plastic pipes and to a process of making such joint, particularly a pipe joint connecting non-shrinkable plastic pipes, in which joint the pipe ends, fittings or the like which are connected tightly to each other under the action of heat and in which the joining sleeve consists of a portion of a pipe end or fitting or of a separate member, which surrounds the gap between the two pipe ends or between the pipe end and the spigot end of the fitting.
b. Description of the Prior Art
Such pipe joints are known under the name of shrink joint or welded joint or shrinkable sleeve or (electrically) weldable sleeve for shrinkable thermoplastic plastic pipes. Such joints can be used to connect shrinkable thermoplastic pipes to each other or to fittings etc., in pipe networks by tight joints, which can be handled conveniently. To make the joint, a separate sleeve is provided around the gap between the elements to be joined so that a small clearance is necessarily left between the inside surface of the sleeve and the outside surface of the pipe ends owing to permissible tolerances of the inside diameter of the sleeve and the outside diameter of the pipe. That clearance is either filled with a liquefied hot-melt adhesive, which supplies to the sleeve the heat required to cause the sleeve to shrink and thus to compress the hot-melt adhesive between the pipe end and the sleeve so as to form a tight joint, as has been proposed in German Patent Specification 2207060, of said clearance is used to accommodate the shrinkage of the sleeve, as has been disclosed is Published German Application 1479231, which teaches to use a heating wire, which is provided with dual insulation and heats the clearance to the required welding temperature, and supplies the sleeve with the heat required for shrinking.
Improvements regarding the arrangement and design of the heating wires for such electrically weldable sleeves for joining conduit pipes consisting of shrinkable thermoplastic plastics have been proposed in German Patent Specification 3103305, which teaches to provide a plurality of resistance heating wires disposed close to the inside surface of the sleeve body, and in German Patent 3,226,575 as well as U.S. Pat. No. 3,235,289, which teach to push a welding ring into the clearance left between the sleeve body and the pipe end and to electrically heat said ring so that it fills the clearance and joins the two parts to each other.
In those known pipe joints either a hot-melt adhesive is used, by which the sleeve body is supplied with the heat required for its shrinkage, or such shrinking heat is supplied by an electrically heated heating coil. Said known processes can only be used for connecting shrinkable thermoplastic materials. The types of joints cannot be used at all with different materials or, e.g., where a clearance between the sleeve and the pipe end is to be filled because they are not sufficiently reliable because a snug contact between the hot-melt adhesive and the surface of the tube is not obtained and because there is no shrinking sleeve for applying pressure.
In connection with non-shrinkable thermoplastic plastics such as polybutene it is possible to use only mechanically acting sleeves, which are very expensive and provide the required safety only when used together with additional fittings, or with welded joints, in which either the end faces of the pipe ends to be joined can be heated above the melting temperature and can then be forced against each other to form a butt-welded joint, or one of the ends to be joined can be designed as a socket and the inside surface of the socket and the outside surface of the pipe can be heated, whereafter said surfaces are pushed one over the other. In both cases the parts to be joined must be moved relative to each other during the welding operation and if the pipes are laid in a confined space the space required for that relative movement is often not available.